The Obama administration’s Russian reset, designed by then-Secretary of State Hillary Clinton, was premised on the idea of Russia as a partner with the United States. Hand in hand, the former rivals would address the major international crises of the day. This initiative will be remembered as one of the biggest foreign policy follies of the modern era — a staggeringly naïve exercise in appeasement that emboldened Moscow at Washington’s expense.

From Damascus to Tehran to Kiev, the Russians have been running rings around a U.S. presidency that believes “leading from behind” is a serious strategy, rather than a policy of surrender.

There can be no doubt that Russian President Vladimir Putin, a man schooled in the Soviet-era KGB, sees Barack Obama as a figure of considerable weakness. With a vacuum of leadership on the world stage, Moscow has gotten its way over Syria, propping up the brutal regime of Bashar al-Assad.

The Kremlin has also successfully enticed the United States to enter into futile, direct negotiations with Tehran over its nuclear program. This buys the mullahs valuable time to advance their nuclear ambitions while building their long-range missile capability.

And now Mr. Putin is massing his forces on the border of Ukraine, threatening the new government in Kiev, convinced that the free world is too weak to stand in his way. Thousands of his soldiers are already on the ground inside the Crimea, a de facto occupying army, hiding under the guise of pro-Russian “self-defense” forces.

In many respects this is a defining moment for the free world in the 21st century and for American leadership in particular. There are those who say this isn’t the West’s fight, that we have no vital interests at stake. They are wrong. Ukraine is at the heart of Europe, bordering four NATO member states. Its 50 million inhabitants share an aspiration to be part of the West.

Allowing Russian tanks to roll into the Crimea with impunity will set an extremely dangerous precedent. Moscow will feel emboldened to enter into other parts of its “near abroad,” intervening under the pretext of protecting ethnic Russians. The Baltic States, with their large Russian-speaking minorities, could be next.

This is no time for a deer-in-the-headlights response from the White House. The world’s superpower must do all it can to warn Russia against an invasion of Ukraine, while bolstering NATO allies in the region.

It is simply not enough to merely talk about expelling Moscow from the G8 — a group that is little more than a glorified talking shop. It would be far more effective to announce the withdrawal of the United States from the New START Treaty. Signed by the Obama administration in 2010, this fundamentally flawed pact hampers Washington’s ability to deploy an effective global missile defense system.

Simultaneously, Washington, as it says it will, should implement targeted sanctions against any Russian officials implicated in aggression against Ukraine, including the freezing of financial assets and the imposition of wide-ranging visa bans. Further, it must be prepared to enforce the Magnitsky Act, passed by Congress in 2012, which restricts travel to the United States for Russian officials implicated in human rights violations.

Additionally, the United States must rally key European allies, including Britain and Germany, to implement similar sanctions against Russia. Such sanctions will hit hard among the Russian elites surrounding Vladimir Putin, many of whom conduct business in the major Western capitals.

But sanctions against Moscow must be coupled with robust support for NATO allies in proximity to Russia and Ukraine. Washington must reassure our NATO allies that their security is guaranteed and the United States should deploy additional military assets to the region to warn Moscow against any expansionist ambitions into NATO territory.

Additionally, U.S. restrictions on the export of liquefied natural gas to NATO partners in Eastern Europe should be eased. This would reduce their energy dependence on Russia.

Barack Obama is clearly no Ronald Reagan but he should take a page from the Gipper’s playbook on global leadership. To be respected on the world stage and exert real influence, the United States must be prepared to stand up to tyrannical regimes that seek to bully those around them while threatening international peace.

President Reagan, together with Margaret Thatcher, brought down the might of the Soviet Empire through a policy of strength and unwavering support for the principles of liberty. The enemies of freedom must be confronted if the free world is to be secure in the 21st century.

Nile Gardiner is director of the Margaret Thatcher Center for Freedom at The Heritage Foundation and a former aide to Lady Thatcher.